Photocoagulators are generally well known in opthalmology for producing a "weld" between the retina and the choroid tissue. The retina is coagulated locally by means of light energy applied to a selected coagulation area. Such instruments use a gas discharge lamp, and preferably a high-pressure xenon arc lamp, to supply the light energy. The instruments have an optical system along a light output path for imaging a luminous plasma region of the lamp on the patient's eye via intermediate images, and a concave mirror behind the lamp reflects light from the luminous plasma region out along the output path to form another image of the luminous plasma region reflected from the mirror. The two images of the plasma region have to be positioned in diametrically opposite relation at the perimeter of the field stop to achieve optimum illumination.
Photocoagulators have interchangeable field stops preferably mounted on a "rekoss" disk so that different sized field stops can be positioned on the output path. Each time the field stop size is changed, the instrument must be adjusted for proper location of the direct and reflected images of the plasma region at the edges of the field stop. In prior art photocoagulators, this required projection of an intermediate image onto a nearby wall and adjustment of the instrument with a wrench in a relatively cumbersome and slow procedure.
The invention involves recognition of the problems of adjusting prior art photocoagulators and realization of a way that adjustment can be made simpler, faster, and more convenient. The invention aims at simplicity, efficiency, reliability, and ease of adjustment in an improved photocoagulator.